Unlike fossil fuels, hydrogen is paid an attention as an ultimate clean energy source generating no substance which is feared in view of an environmental problem such as carbon dioxide gas and sulfur oxides even when fired, the calorie per unit mass of hydrogen is three times the colories of a petroleum, and when hydrogen is supplied to a fuel cell, it can be converted into electric energy and thermal energy at high efficiency.
For producing hydrogen, technique such as a method for thermal decomposition of water and steam-reforming of natural gas or naphtha has previously been proposed as a chemical process. Since this process requires the reaction conditions at high temperatures and pressures, and the synthetic gas produced contains CO (carbon monoxide), it becomes necessary to perform CO removal which is technically solved with difficulty, so as to prevent the deterioration in a fuel cell electrode catalyst, when such hydrogen is used as a fuel for fuel cells.
On the other hand, in a biological hydrogen production method using a microorganism, it is not necessary to remove CO, because such method has the reaction conditions at normal temperatures and pressures, and the generated gas does not contain CO.
From these aspects, biological hydrogen production using a microorganism is more preferable as a method of supplying a fuel for fuel cells.
Although the biological hydrogen production method has such excellent characteristics, a great progress has not been previously made as a method of supplying a fuel for fuel cells because such method has no economical practicability due to the productivity of hydrogen production, particularly a low hydrogen-generation rate (STY; Space Time Yield) per unit volume.
Biological hydrogen production methods are roughly classified into a method using a photosynthesis microorganism, and a method using a non-photosynthesis microorganism (mainly anaerobic microorganisms). Since the former method uses light energy for hydrogen generation, there are many problems to be solved such as cost of hydrogen generation apparatuses requiring a large light collecting area due to its low light energy utilization efficiency, and difficult maintenance and management, and this method is not at practical level.
The latter conventional hydrogen production method using an anaerobic microorganism relies on division and proliferation of the anaerobes. Anaerobic microorganisms have extremely slow division and proliferation (U.S. Pat. No. 5,834,264, and R. Nandi et al., Enzyme and Microbial Technology 19:20-25, 1996), and division and proliferation of anaerobic microorganisms require a greater free space necessary upon division and proliferation (“space” necessary for culturing, that is, proportionate to a reactor volume) as compared with that of other microorganisms although the reason has not been clarified. For this reason, the cell concentration in the stationary state that can be achieved by culturing of anaerobic microorganisms under anaerobic conditions necessary for hydrogen generation in the “space” of a fixed size is absolutely low as compared with that of other microorganisms. For these reasons, a hydrogen generation rate (STY) of anaerobic microorganisms is not sufficient. In this respect, significant improvement is demanded.
In addition, when hydrogen produced by a biological production method is supplied to a fuel cell of a constant electric capacity, it is practically necessary to make a supply of an organic substrate as a hydrogen source to a hydrogen generation reactor, and a rapid response of current generation. Also in this respect, technical solution is demanded.